1. Field of Invention:
The present invention relates to a mechanism for adjusting the force on a stylus in a profiler.
2. Description of the Prior Art:
Stylus profilers and other step height measurement instruments are devices for measuring the profile or contour of a surface. They are used in the testing of the surface of semiconductor devices and in metal working, chemical, printing, information storage and other industries. The common technique for manufacturing semiconductors, for example, includes depositing certain materials on a substrate or wafer. A profiler measures the pattern and dimension of the deposits which are crucial for the desired operation of the semiconductors. In other applications the stylus may be used to characterize the primary surface itself.
Both a profiler and other step height measurement instruments each rely on a narrow-diameter stylus moving along a surface either by movement of the stylus or movement of the surface of interest. Stylus movement should be linear to result in a true profile. In this specification, the term "profiler" generally means a device in which the stylus and the surface of interest move linearly with respect to each other. There are instruments, called "step height measurement instruments" here, in which the stylus moves in an arc. As the stylus encounters surface features, the stylus moves vertically to accommodate the deposits, irregularities or finish. Typically, the stylus is mounted on an arm, and the arm is coupled to a linear variable differential transformer (LVDT). LVDT's are electromechanical transducers that produce electrical outputs proportional to the displacement of a core movable in a primary and secondary coil of a transformer. The core is coupled to the stylus arm, and vertical movement of the stylus moves the stylus arm, which in turn moves the coil in the transformer to modify the output voltage. This output voltage is read, and the height and location of features on the surface is determined at locations which are read by devices associated with the carriage.
Accurate measurement of very small features becomes more crucial as, for example, more circuit elements are deposited on semiconductors. It becomes more desirable, therefore, to use a narrower diameter stylus; wider diameter styli introduce too much error. As the stylus diameter decreases, if the downward force is held constant, the force per unit area (pressure) increases as a function of the square of the decrease in diameter. Unless the force transmitted to the surface from the stylus is carefully controlled, the stylus could damage the surface or surface feature, and the stylus itself can be overloaded.
In the past, stylus force was controlled by means of a spring-balance or counter-balance arrangement. That is, the weight of the arm-stylus combination is balanced by an upward spring force and/or a mass counter-balance. Spring force varies with position and, therefore, may yield errors. Because of the manner in which the arm is attached for pivoting on the profiler body, a spring may not act linearly. There is further concern that the spring constant will change even though profilers operate in controlled environment.
In other fields such as phonograph record arms, magnets have been used for balancing and controlling the stylus pressure. Examples include Nakajima, U.S. Pat. No. 4,138,121 (1979) and Rabinow, U.S. Pat. No. 3,830,505 (1974). These devices balance a stylus in the 0.5-2.0 gram range. Stylus profilers, however, operate with much more critical force adjustments, in the milligram range, and the force must remain constant as the stylus moves vertically.